Studies of Mixed-Anion Manganites and Other Compounds

Abstract

Relative to oxides, mixed anion materials containing N and/or F dissolved within an oxide lattice offer a variety of advantages, including smaller band gaps and incorporating a greater variety of potential cations with a larger possible range of oxidation states. These factors, coupled with the relatively lowered symmetry that may occur due to anion ordering suggest enhanced dielectric, magnetic, and/or optical properties for the targeted materials. The goal of this project is to synthesize a mixed-anion analog of (Ca, La)MnO3, which is well known for colossal magnetoresistive properties due in part to the presence of both Mn3+ and Mn4+ in the lattice. Such materials may have potential applications in magnetic storage devices (e.g. computer hard drives). Whereas the presence of mixed Mn3+/4+ ions results from substituting La3+ for some Ca2+ in the oxide lattice, the same effect can hypothetically be achieved through anion substitutions such as N3- and/or F- for O2-. Attempts were made in this study to directly synthesize novel CaMnNxOyFz materials from binary Mn and Ca compounds, or from redox reactions in N2, but they were unsuccessful. These attempts will be summarized and a new approach involving ammonolysis of CaMnO3, followed by fluorination of the product to give the novel compound Ca2(Mn3+, Mn4+)O5F, will be discussed. A second project will also be discussed involving crystal chemical studies of Sr2BiGa11O20. The single crystal X-ray structure of the compound, which is found to be isostructural with Ba2BiGa11O20, is presented for the first time. The structure will also be discussed in the context of a bond-valence sum analysis.